1. Field of the Invention
The present invention relates to an improvement of a drive system of a hybrid vehicle.
2. Description of Related Art
There is known a hybrid vehicle including: a differential device which comprises a first differential mechanism and a second differential mechanism and which comprises four rotary components; an engine, a first electric motor, a second electric motor and an output rotary member which are respectively connected to the four rotary components; and a plurality of coupling elements which are provided to selectively connect the rotary components to each other or to a stationary member, to establish a selected one of a plurality of electric motor drive modes and a plurality of constant-speed-ratio engine drive modes. JP-2011-98712 A1 discloses an example of such a hybrid vehicle which is arranged to establish a selected one of a plurality of electric motor drive modes, a plurality of hybrid drive modes and a plurality of constant-speed-ratio engine drive modes, according to a selected one of different combinations of operating states of the coupling elements.
The prior art hybrid vehicle described above is required to be switched to one of the electric motor drive modes when a predetermined condition to establish this electric motor drive mode is satisfied, or when an EV switch (electric motor drive selector switch) is operated, while the hybrid vehicle is placed in one of the constant-speed-ratio engine drive modes in which the engine is used as a vehicle drive power source, for example, in one of a first-speed drive mode, a second-speed drive mode, a third-speed drive mode and a fourth-speed drive mode. However, the prior art hybrid vehicle has a problem of complicated and time-consuming drive mode switching operations involving repeated changes of the operating states of selected ones of the plurality of coupling elements in the form of clutches and brakes, due to lack of consistent procedures to establish the selected one of the different drive modes.